Introduction

This document explores the expert opinion gathered from staff at the Woodland Trust, related to the measurement of 14 indicators of woodland ecological condition, and the relationship between these indicators’ and a wood’s potential to promote flourishing wildlife species communities (by estimating “value functions” and importance weightings for each indicator).

Expert opinion on the value function and weight for each indicator was elicitated using the Delphi method. The Delphi method is designed to collect and distill expert knowledge, using repeated surveys to refine estimates, increasing their reliability and identifying where agreements and disagreements in understanding occur.

The method relies on repetitive surveying in at least two to three ‘rounds’ where participants are asked the same questions. After each round participants review and consider the answers given by all panel-members, and are then given the opportunity to revise their answers. The Delphi method has a history of successful use in supporting conservation action where empirical evidence is insufficient and information is needed rapidly to inform decisions. It’s transparency and repeatability also add to its appeal.

Each participant either attended an introductory workshop to the process, or was taken through individually. A sub-section of participants were also involved in development the proposed methods to measure each indicator in the field. Answers were submitted into individual Excel spread sheets, with questions relating to:

  • Value functions describing how each indicator related to WEC, in typical conditions
  • Weightings decreeing the relative important of each indicator to WEC, in typical conditions
  • Certainty around value function and weighting estimates (subjective score 0-5)
  • Comments or concerns relating to: situations where these “typical” responses might be inappropriate, the proposed method to measure indicators, and any other relevant information.

Participants were encouraged to keep a clear distinction between the quantitative responses provided (i.e. value functions and weightings under typical conditions) and exceptions to those typical relationships (that they highlighted in their comments).

It was also stated that noted exceptions would be analysed and, where appropriate, used to inform future development of the WEC measure to account for important nuance.

Indicators considered

Analysis Method

Scaling

Values and weights are all measured relative to each other. Some respondents did not use the full 0 to 100 range for all value functions and weights. Comparison between participants requires that they all use the same scale, and so all value functions where scaled from 0 to 100 and weightings were scaled so that the highest indicator weighting for each panelist was 100.

Value functions scaled for:

- Adam Th - Number deadwood classes present (of 3 potential classes) across 4 plot-quarters,
- Adam Th - Number of native tree and shrub species,
- Adam Th - Number of tree size classes (age proxy),
- Adam Th - Vertical structure: Number of tree and shrub canopy layers present,
- Alasdair Fi - Number deadwood classes present (of 3 potential classes) across 4 plot-quarters,
- Alasdair Fi - Number of tree size classes (age proxy),
- Alasdair Fi - Tree regeneration,
- Alasdair Fi - Vertical structure: Number of tree and shrub canopy layers present,
- Chris R - Number of ancient/veteran trees per 1 ha plot,
- Chris R - Number of native tree and shrub species,
- Chris R - Number of tree size classes (age proxy),
- Chris R - Vertical structure: Number of tree and shrub canopy layers present,
- Dave Bo - Extent/ area of woodland,
- Dave Bo - Number deadwood classes present (of 3 potential classes) across 4 plot-quarters,
- Dave Bo - Number of ‘positive indicator’ plants per plot (10m radius circle),
- Dave Bo - Number of ancient/veteran trees per 1 ha plot,
- Dave Bo - Number of native tree and shrub species,
- Dave Bo - Number of tree size classes (age proxy),
- Dave Bo - Tree regeneration,
- Dave Bo - Vertical structure: Number of tree and shrub canopy layers present,
- Emma G - Number of native tree and shrub species,
- Emma G - Number of tree size classes (age proxy),
- Emma G - Occupancy of native trees & shrubs in all canopy layers,
- Jim Sm-Wr - Extent/ area of woodland,
- Jim Sm-Wr - Horizontal complexity (structural mosaics across a wood),
- Jim Sm-Wr - Invasive plant species presence and cover,
- Jim Sm-Wr - Number of ‘positive indicator’ plants per plot (10m radius circle),
- Jim Sm-Wr - Number of native tree and shrub species,
- Jim Sm-Wr - Number of tree size classes (age proxy),
- Jim Sm-Wr - Occupancy of native trees & shrubs in all canopy layers,
- Jim Sm-Wr - Vertical structure: Number of tree and shrub canopy layers present,
- Kylie Jo-Ma - Extent/ area of woodland,
- Kylie Jo-Ma - Herbivore impact,
- Kylie Jo-Ma - Number of ‘positive indicator’ plants per plot (10m radius circle),
- Kylie Jo-Ma - Number of ancient/veteran trees per 1 ha plot,
- Kylie Jo-Ma - Number of native tree and shrub species,
- Kylie Jo-Ma - Number of tree size classes (age proxy),
- Kylie Jo-Ma - Occupancy of native trees & shrubs in all canopy layers,
- Kylie Jo-Ma - Tree regeneration,
- Kylie Jo-Ma - Vertical structure: Number of tree and shrub canopy layers present,
- Liam Pl - Extent/ area of woodland,
- Liam Pl - Herbivore impact,
- Liam Pl - Number of native tree and shrub species,
- Liam Pl - Number of tree size classes (age proxy),
- Liam Pl - Tree regeneration,
- Liam Pl - Vertical structure: Number of tree and shrub canopy layers present,
- Lou Ha - Number of tree size classes (age proxy),
- Lou Ha - Vertical structure: Number of tree and shrub canopy layers present,
- Martin Hu - Horizontal complexity (structural mosaics across a wood),
- Martin Hu - Number deadwood classes present (of 3 potential classes) across 4 plot-quarters,
- Martin Hu - Number of native tree and shrub species,
- Martin Hu - Occupancy of native trees & shrubs in all canopy layers,
- Mick Br - Anthropogenic damage,
- Mick Br - Herbivore impact,
- Mick Br - Horizontal complexity (structural mosaics across a wood),
- Mick Br - Invasive plant species presence and cover,
- Mick Br - Number deadwood classes present (of 3 potential classes) across 4 plot-quarters,
- Mick Br - Number of ‘positive indicator’ plants per plot (10m radius circle),
- Mick Br - Number of ancient/veteran trees per 1 ha plot,
- Mick Br - Number of native tree and shrub species,
- Mick Br - Number of tree size classes (age proxy),
- Mick Br - Occupancy of native trees & shrubs in all canopy layers,
- Mick Br - Tree disease and rapid mortality,
- Mick Br - Tree regeneration,
- Peter Lo - Extent/ area of woodland,
- Peter Lo - Herbivore impact,
- Peter Lo - Horizontal complexity (structural mosaics across a wood),
- Peter Lo - Number of ‘positive indicator’ plants per plot (10m radius circle),
- Peter Lo - Number of ancient/veteran trees per 1 ha plot,
- Peter Lo - Number of tree size classes (age proxy),
- Peter Lo - Vertical structure: Number of tree and shrub canopy layers present,
- Rich Br - Number of tree size classes (age proxy),
- Rich Br - Vertical structure: Number of tree and shrub canopy layers present,
- Saul H - Extent/ area of woodland,
- Saul H - Horizontal complexity (structural mosaics across a wood),
- Saul H - Number of tree size classes (age proxy),
- Saul H - Vertical structure: Number of tree and shrub canopy layers present,
- Vanessa B - Herbivore impact,
- Vanessa B - Number of tree size classes (age proxy),
- Vanessa B - Vertical structure: Number of tree and shrub canopy layers present

Weights were scaled for:


- Chris R,
- Dave Bo,
- Hannah Pa,
- Iain Mo,
- Jim Sm-Wr,
- Liam Pl,
- Saul H

Edits to responses

In some cases - where appropriate to allow categorisation - points were added to value functions by visual interpolation. Some points where also added by extrapolation where a maximum/minimum value was clearly implied.

Displaying of results

A copy of this report was circulated to respondents between rounds to allow respondents to investigate estimates provided by others, and their certainties.

Where appropriate value functions where plotted categorically (with box plots summarising the estimated value at each category), and continuously (with lines connecting points and an indication of the general trend - see model details below). Indicator relative weightings were presented on boxplots, including the mean and median estimates. Interactive hover-text displayed respondent name and their certainty in their estimate.

Model details

Value functions were modeled with a binomial GAM. The influence of points from each respondent weighted by the inverse of the number of points provided by that respondent, with the aim of ensuring that all respondents had the same influence on the result, regardless of how many points they provided.

Participants and survey completion

25 expert practitioners from Woodland Trust and Plantlife staff were invited to provide their opinion the value and weight of indicators.
20 have provided a response as of Wed Nov 22 16:31:25 2023.
Of these, 12 remain partially incomplete.

Survey completion summary

The chart below summarises participant completion of different elements of the survey. For more detail see table (button below chart).

Figure 1. Completion of expert opinion survey components by each panelist (coloured portion of pies) for value functions (“Value func”), value function certainties (“VF cert”), indicator weightings (“Weights”) and weighting certainties (“wt certs”).

Survey completion details

Reponses by indicator

Tree age/size distribution

[1] “Number of tree size classes (age proxy)” [1] “17 respondants.”

Categorised value function

measure Mean Standard deviation
1 0.0 0.0
2 40.0 15.5
3 76.5 15.0
4 100.0 0.0

Continuous Value function

Weights

Comments

Consensus on VF likely:
- All continuous relationships have same shape.
- Complete agreement on upper and lower value.
- Some put maximum value at 3 classes.

Disagreement on weight:
- Two camps, giving relatively low and high weights (compared to other indicators) at c. 60 and 90, respectively.

Canopy nativness

[1] “Occupancy of native trees & shrubs in all canopy layers” [1] “17 respondants.”

Continuous Value function

Weights

Comments

VF:
- Similar trends. Some prefer linear, some S-shaped, some exponential, some logarithmic - Bit of disagreement RE requirement for max value

Weightings: fairly wide spread

Vertical Structure

Of four possible categories: >15m; 5-15m; 2-5m; 0.5-2m; measured at plot level

[1] “Vertical structure: Number of tree and shrub canopy layers present” [1] “15 respondants.”

Categorised value function

measure Mean Standard deviation
0 0.0 NA
1 1.7 6.5
2 43.9 21.9
3 86.9 10.3
4 100.0 0.0
5 100.0 NA

Continuous Value function

Weights

Comments

Native tree an shrub species richness

[1] “Number of native tree and shrub species” [1] “17 respondants.”

Categorised value function

measure Mean Standard deviation
0 0.0 0.0
1 1.5 4.1
2 11.2 9.6
3 35.6 16.0
4 49.0 22.9
5 80.2 16.3
6 80.7 24.5
7 93.8 7.3
8 87.4 18.1
9 97.3 4.6
10 93.8 8.9
11 98.0 2.8
12 98.6 3.4
13 99.0 1.4
14 99.9 0.3
15 100.0 0.0

Continuous Value function

Weights

Comments

Invasives

[1] “Invasive plant species presence and cover” [1] “17 respondants.”

Categorised value function

measure Mean Standard deviation
0 100.0 0.0
1 55.0 7.1
5 58.2 36.3
10 66.5 30.1
15 90.0 NA
20 39.4 22.9
25 34.3 33.9
30 29.1 25.7
35 45.0 NA
40 16.4 16.9
45 10.0 14.1
50 16.1 23.0
55 12.0 NA
60 7.5 11.8
65 8.0 NA
70 9.2 14.2
75 5.0 NA
80 4.9 7.1
85 3.0 NA
90 3.0 3.9
95 1.0 NA
100 0.0 0.0

Continuous Value function

Weights

Comments

Deadwood

[1] “Number deadwood classes present (of 3 potential classes) across 4 plot-quarters” [1] “15 respondants.”

Categorised value function

measure Mean Standard deviation
0 0.0 0.0
1 11.1 10.8
2 19.0 12.7
3 43.5 20.5
4 41.5 22.0
5 68.0 31.5
6 67.9 25.1
7 72.7 23.7
8 79.7 26.1
9 85.0 11.8
10 91.3 15.7
11 97.3 4.6
12 97.3 10.3

Continuous Value function

Weights

Comments

AVTs

[1] “Number of ancient/veteran trees per 1 ha plot” [1] “15 respondants.”

Categorised value function

measure Mean Standard deviation
0.0 0.0 0.0
0.5 26.0 NA
1.0 38.4 20.4
1.5 66.0 NA
2.0 47.3 23.9
2.5 86.0 NA
3.0 77.5 17.2
3.5 100.0 NA
4.0 83.2 25.5
5.0 84.0 NA
6.0 80.1 17.8
7.0 91.0 NA
8.0 92.4 10.0
9.0 96.0 NA
10.0 97.3 4.5
11.0 99.0 NA
12.0 98.0 6.3

Continuous Value function

Weights

Comments

Woodland Extent

[1] “Extent/ area of woodland” [1] “13 respondants.”

Categorised value function

measure Mean Standard deviation
0 0.0 0.0
1 3.8 4.8
2 5.5 7.8
3 24.0 NA
5 21.9 17.2
8 74.0 NA
10 40.3 23.7
15 64.5 12.0
20 70.6 25.2
25 70.7 18.1
30 82.1 22.5
35 91.0 NA
40 89.7 15.0
45 97.0 NA
50 100.0 0.0

Continuous Value function

Weights

Comments

Regeneration

[1] “Tree regeneration” [1] “15 respondants.”

Categorised value function

measure Mean Standard deviation
0 0.0 0.0
1 38.9 33.2
2 71.0 24.5
3 100.0 0.0

Continuous Value function

Weights

Comments

Herbivore Impact

[1] “Herbivore impact” [1] “14 respondants.”

Categorised value function

measure Mean Standard deviation
0 55.2 44.5
1 83.2 30.9
2 55.2 27.6
3 36.5 33.0
4 22.2 42.2

Continuous Value function

Weights

Comments

Tree Health

[1] “Tree disease and rapid mortality” [1] “14 respondants.”

Categorised value function

measure Mean Standard deviation
0 82.2 34.5
1 0.0 NA
5 65.0 49.5
10 76.0 29.5
15 30.0 14.1
20 47.0 30.8
25 61.7 38.2
30 36.2 39.0
40 33.9 33.9
50 32.5 23.6
60 26.0 31.5
70 22.5 10.6
75 25.0 NA
80 20.1 36.0
90 7.5 3.5
100 7.1 26.7

Continuous Value function

Weights

Comments

Ground Flora

[1] “Number of ‘positive indicator’ plants per plot (10m radius circle)” [1] “14 respondants.”

Categorised value function

measure Mean Standard deviation
0 0.0 0.0
1 13.3 10.4
2 21.2 20.2
3 35.8 43.0
4 41.7 7.6
5 41.7 12.2
6 70.0 14.1
7 66.5 30.4
8 90.0 10.0
9 92.0 NA
10 85.0 11.8
11 94.0 NA
12 87.5 10.6
13 96.0 NA
14 97.0 NA
15 95.1 5.5
16 99.0 NA
17 97.5 3.5
18 100.0 NA
19 100.0 NA
20 100.0 0.0

Continuous Value function

Weights

Comments

Horizontal complexity

[1] “Horizontal complexity (structural mosaics across a wood)” [1] “13 respondants.”

Categorised value function

measure Mean Standard deviation
0 0.0 0.0
1 5.5 9.6
2 32.9 13.1
3 70.1 21.0
4 89.4 12.7
5 100.0 0.0

Continuous Value function

Weights

Comments

Human Damage

[1] “Anthropogenic damage” [1] “16 respondants.”

Categorised value function

measure Mean Standard deviation
0 93.8 25.0
2 50.0 NA
5 70.0 34.6
10 71.5 33.4
15 27.5 38.9
20 35.6 27.9
25 51.7 28.4
30 28.3 36.9
35 10.0 NA
40 27.9 22.0
45 4.0 NA
50 24.7 21.8
55 2.0 NA
60 16.6 22.7
61 0.0 NA
65 0.0 NA
70 21.7 25.7
75 11.7 12.6
80 15.0 31.9
85 0.0 NA
90 1.0 1.4
95 0.0 NA
100 6.7 25.8

Continuous Value function

Weights

Comments

plot all indicators sequentially

Tree age distribution

Native canopy percentage

Vertical structure

N tree & shrub spp.

Invasive plants % cover

Deadwood

Veteran trees

Woodland extent

Regen

Herbivore damage

Tree health

Ground flora

Horizontal complexity

Anthropogenic damage

Indicatior weights

Certainties

Discussion points

  • - Some value functions have a minimum score >0. Strictly speaking this is okay… but it does meant that the final condition score will not be on a 0-100 scale (there will always be some minimum condition score).